PLEASE NOTE: As of 9/24/09, I have updated this post for WOTLK. I'm leaving this here for posterity, but please go visit the updated post here.
As a raiding mage, you have to know what your role in a raid is. You're not there to heal or to tank (although you do tank in some encounters). You're not there because everybody loves the intellect buff and your free food and water. You're there to do damage, and lots of it. We've got a little bit of all the flavors of damage as well. Aoe? Oh yeah... we're the kinds. Burst damage? Yeah, we got it. Sustained damage? I suppose this is kind of spec dependent, but there's certainly builds that help keep this even through most of the long fights. DoT's? Well... umm... Not exactly. But if you can get a Rolling Ignite and crit every 4 seconds, you're causing some serious damage. Okay, okay, we're not kings all around, but that wouldn't really be fair to the other classes, so we should be happy with what we've got. (And I am.)
"But which spell do I use in each situation?" you might ask. Being a deep fire spec (10/48/3), there's a few things that you should do to keep your DPS at it's height. The main thing is having 5 scorch (with improved scorch talent) up on your target at all times. This will increase all damage from fire spells by 15%. So, as far as Spell Rotation on bosses go, people say to start out with 5 scorches, and then something like 7 fireballs, 1 scorch, 7 fireballs, 1 scorch... etc. This keeps you doing your main nuke (fireball) for most of the time, but including a scorch within that 30 seconds to keep the full 5 debuffs on the boss. Personally, I don't buy it. I think it's kinda silly. What if you get resisted? You don't have time to cast another scorch before the debuff wears off. So, let's look at what each of the direct damage fire spells do so we can understand better when to use them. (I based the numbers from Arcannis Mage Damage and Talent Calculator using my current spec and gear bonuses.)
Fireball- This is your nuke. You're going to be casting this spell most of the time. Besides Pyroblast, it is your hardest hitting spell, but it is your highest DPS spell that does not have a cooldown. My average hit is about 2723 with a DoT of about 222 damage over 8 seconds. The average DoT from Incinerate is about 261 damage for 4 seconds. I have a sustained DPS (meaning over time and factoring in crits) of about 993 with this spell. DPM (Damage per Mana) comes in at 11.03. It's bread and butter; use it.
Scorch- As I mentioned before, if you have Improved Scorch as a talent, you want to make sure you have 5 of these debuffs on your target at all time. The debuff lasts for 30 seconds and is renewed every time somebody hits the target with another improved Scorch. (Remember, "hits" is an important word. The spell can be resisted.) With multiple mages, you can get 5 debuffs on a target in no time. The spell doesn't hit as hard as Fireball, so you can usually start a little earlier on the initial pull (don't have to worry about pulling agro as much). The DPS is 849 with a DPM of 11.45. The higher DPM means that you won't run out of mana as fast as with Fireball, but you won't do as much damage since the DPS is lower.
Fire Blast- This is the highest DPS single-target spell for mages. The hits aren't the strongest, but since it's an instant cast, the DPS comes in about 1334. The DPM, however, is 6.84. If you could chain cast this spell, you'd be out of mana real quick. But with the 8 second cooldown (6.5 talented), you'd be sitting around a lot. Use this spell if you need some burst damage, or you have some available mana. It's usually threaded somewhere in the middle of Fireballs.
Pyroblast- This is your hardest hitting spell. My average Pyroblast hit is about 3238 damage. But the super long cast time makes the DPS only 676. There's not a lot of reasons to use this in the middle of battle. If you've got Presence of Mind in your spec, an instant-Pyroblast is a huge boost to Pyroblast. It's also not a bad way to open with this spell. The DoT from this spell is the strongest we've got, so it doesn't hurt to have it on in the beginning of a fight. Let the tanks get agro while we wind up with one of these bad boys. It's even nicer if you have another fire mage in your group that can get the scorches up while you're casting this monster. It's deadly, no doubt, but, like I said, you probably don't want to be chain casting it.
That's about it for this week's Single Target Fire Spells lesson. I'll go through other specs and spells as well in due time. Our Kara group succeeded in clearing the tower in 2 days, so I actually have a few days of not being in there. :)